In general, a complementary metal oxide semiconductor (CMOS) ring oscillator has a wider modulation region without using a larger manual device, thereby is used widely in the applications such as wireless communication. Currently, with the development of the CMOS technology, the CMOS ring oscillator is used in the RF applications such as broadcasting tuners, GPS receivers and wireless LAN transceivers (WLAN).
However, the noise performance of the CMOS ring oscillator is weaker, and thus it has limitations, such as the problem of high sensitivity for the variation of temperature and power supply.
Currently, for solving the problem of high sensitivity for the changed rate of temperature and power supply, a band-gap reference voltage circuit which provides a modulated reference voltage according to the variation of temperature and power supply to the ring oscillator is supplied. That is, when the temperature increases, the frequency of the ring oscillator decreases, and the band-gap reference voltage circuit in positive proportion to the temperature is used to raise the frequency thereof for temperature compensation.
Refer to FIG. 1. FIG. 1 is a block diagram showing a ring oscillator of a conventional band-gap reference voltage circuit generating an oscillation signal.
Referring to FIG. 1 again, a band-gap reference voltage circuit 10, a linear regulator 20, a ring oscillator 30 and a level shifter 40 are shown, wherein the band-gap reference voltage circuit 10 provides a reference voltage by using an external applied power supply, and the linear regulator 20 regulates and outputs the reference voltage according to a constant voltage, and the ring oscillator 30 oscillates and generates a pulse train according to the regulated reference voltage, and the level shifter 40 shifts the pulse train which is generated by the ring oscillator 30 according to a constant level, and outputs it.
The band-gap reference voltage circuit 10 includes an amplifying terminal using the external power voltage to performing amplifying, and provides the reference voltage (VREF) according to the output value of the amplifying terminal. At this time, for temperature compensation, the reference voltage (VREF) generated by the band-gap reference voltage circuit 10 has a value which varies according to the slope of a temperature coefficient (TC).
That is, for compensating the frequency of the ring oscillator which is decreased due to the increased temperature, the band-gap reference voltage circuit 10 with positive-TC increases the reference voltage (VREF), thereby increasing the voltage (VDDO) applied to the ring oscillator.
The linear regulator 20 receives the reference voltage (VREF) generated by the band-gap reference voltage circuit 1 for temperature compensation, and outputs the reference voltage (VREF) in the constant voltage (VDDO) with a constant ratio.
The ring oscillator 30 is composed of an odd number of inverters which are connected in a ring-shape, and is driven by the constant voltage (VDDO) of the linear regulator 20, thereby outputting the pulse train with a constant frequency. At this time, the ring oscillator 30 uses the reference voltage (VREF), which is generated after temperature compensation, to generate an oscillation clock for compensating the frequency.
The level shifter 40 appropriately shifts a direct current (DC) voltage level of the signal generated by the ring oscillator 30 and outputs to the RF receiver including the ring oscillator 30.
The band-gap reference voltage circuit with positive-TC has an output noise of low level and PSRR of high level. However, since the ring oscillator is sensitive to the variation of the driving voltage (VDDO), the band-gap reference voltage circuit 10 is sensitive to 1/f noise and thermal noise, and has the problem of a degradation of noise performance. Furthermore, the noise performance will directly impact the ring oscillator 30. Therefore, the frequency of the ring oscillator 30 is varied by the variation of temperature and power supply, resulting in the problem that the oscillation signal can not be generated accurately.
Again, since the band-gap reference voltage circuit 10 includes the amplifying terminal, it results in a noise amplification phenomenon that the current and the voltage therein are amplified at the same time, thereby worsening the noise performance.